Methods for producing syngas from H2S and CO2 in an electrochemical cell

ABSTRACT

The present application provides systems, apparatuses, and methods for simultaneous processing of tow waster gases, namely H2S and CO2. In an exemplary process of this disclosure H2S is supplied to anode side of an electrochemical cell, while CO2 is supplied to the cathode side. As a result, valuable commercial products are produced. In particular, SO2 is harvested from the anode side, while synthesis gas, CO+H2) is harvested from the cathode side. An electric current is also produced, which can be supplied to a local utility grid.

TECHNICAL FIELD

This disclosure relates to a combination of solid oxide fuel cell and anelectrolysis cell to produce valuable commercial products fromindustrial waste gases. In particular, the disclosure relates toeffectively reacting H₂S and CO₂ in an electrochemical cell to produceSO₂, syngas, and/or an electric current.

BACKGROUND

With the rising concerns of increasing CO₂ emissions and the resultantglobal warming and related climate changes, governments and companiesaround the world are looking for ways to reduce their energy intensityand carbon footprint. One way to reduce CO₂ emissions is to userenewable energy sources, such as solar, wind, and geothermal energysources. However, despite decreasing costs of “green” energy fromrenewable sources, the amount of energy generated from all such sourcescombined is insufficient to satisfy the growing global demand forenergy, and the anthropogenic CO₂ emissions continue to rise, addingbillions of tons of CO₂ to the atmosphere every year. In addition,various industrial processes, petrochemical industry in particular,produce billions of tons of H₂S. Each year the U.S. alone produces tensto millions of tons of H₂S, mainly as a by-product obtained duringrefining of fossil fuels. Due to lack of commercially viable use for theH₂S itself, nearly all of it is converted to elemental sulfur in theClaus process, and the elemental sulfur is then stored in open fields asa waste. Economically efficient utilization of either or both of CO₂ andH₂S is a formidable challenge.

SUMMARY

The present disclosure is based, at least in part, on a realization thatusing an electrochemical cell including an anode, a cathode, and ahybrid electrolyte membrane, the waste gases CO₂ and H₂S can beconverted to valuable products. In particular, the disclosure is basedon integrating (i) a solid oxide fuel cell (“SOFC”), where the chemicalenergy of a fuel (H₂S) is converted into electrical energy and avaluable side-product SO₂ is produced, and (ii) an electrolysis cell,where the electricity generated by SOFC is used for electrolysis of CO₂and H₂O to yield synthesis gas, which is a mixture of CO and H₂.

In one general aspect, the present disclosure provides a method forproducing syngas (CO and H₂) and SO₂ from H₂S, CO₂, and H₂O in anelectrochemical cell containing an anode, a cathode, and a conductivemembrane positioned between and in electrochemical contact with theanode and the cathode, the method including the following steps:

contacting the anode of the electrochemical cell with a streamcontaining mainly H₂S and H₂O to produce a stream containing mainly SO₂;and

contacting the cathode of the electrochemical cell with a streamcontaining mainly CO₂ and H₂O to produce a stream containing mainlysyngas.

In some embodiments, the method includes contacting the anode andcontacting the cathode occur simultaneously.

In some embodiments, the method further includes collecting the streamcomprising SO₂ from the anode side of the electrochemical cell.

In some embodiments, the method further includes collecting the streamcomprising the syngas from the cathode side of the electrochemical cell.

In some embodiments, the stream comprising H₂S and H₂O is a gas.

In some embodiments, the pressure of the stream is from about 1 bar toabout 20 bar.

In some embodiments, the temperature of the stream is from about 100° C.to about 1,500° C.

In some embodiments, molar ratio of H₂S to H₂O in the stream is fromabout 0.1:1 to about 10:1.

In some embodiments, temperature of the stream is from about 700° C. toabout 1,000° C.

In some embodiments, the anode material comprises WS₂, CoS_(1.035) orLi₂S.

In some embodiments, the stream comprising CO₂ and H₂O is a gas.

In some embodiments, the pressure of the stream is from about 1 bar toabout 20 bar.

In some embodiments, the temperature of the stream is from about 100° C.to about 1,500° C.

In some embodiments, molar ratio of CO₂ to H₂O in the stream is fromabout 0.1:1 to about 10:1.

In some embodiments, temperature of the stream is from about 700° C. toabout 1,000° C.

In some embodiments, the cathode material comprises a perovskitematerial of the general type La_(1-x)Sr_(x)Cr_(1-y)M_(y)O₃, where M is ametal selected from Mn, Fe, Co, and Ni.

In some embodiments, the conductive membrane comprises a materialselected from yttria-stabilized zirconia (YSZ), scandia stabilizedzirconia (ScSZ), gadolinium doped ceria (GDC), lanthanum strontiumcobalt ferrite (LSCF), Sr-doped La manganites (LSM), andferrites-nickelates (LSFN).

In some embodiments, molar ratio of H₂S to CO₂ in the electrochemicalcell is from about 0.1:1 to about 10:1.

In some embodiments, molar ratio of CO to H₂ in the syngas is from about1:1 to about 1:5.

In some embodiments, the syngas comprises no more than from about 0.5wt. % to about 10 wt. % of CO₂ or H₂O, or a combination thereof.

Unless otherwise defined, all technical and scientific terms used hereinhave the same meaning as commonly understood by one of ordinary skill inthe art to which the present application belongs. Methods and materialsare described herein for use in the present application; other, suitablemethods and materials known in the art can also be used. The materials,methods, and examples are illustrative only and not intended to belimiting. All publications, patent applications, patents, sequences,database entries, and other references mentioned herein are incorporatedby reference in their entirety. In case of conflict, the presentspecification, including definitions, will control.

Other features and advantages of the present application will beapparent from the following detailed description and FIGURES, and fromthe claims.

DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic description of the electrochemical process ofproducing SO₂, CO, and H₂ from H₂S, CO₂, and H₂O.

DETAILED DESCRIPTION

Generally, the present disclosure related to systems, apparatuses, andmethods to effectively react waste gases CO₂ and H₂S, in the presence ofH₂O, to produce syngas (a mixture of mainly CO and H₂), SO₂, and adirect electric current, all of which are valuable industrial products.In one aspect, the disclosure provides an electrochemical cell, whichincludes an anode, a cathode, and a conductive membrane positionedbetween and in electrochemical contact with the anode and the cathode.The disclosure further provides a method of using said electrochemicalcell to produce SO₂, syngas, extra H₂, and an electric current, fromCO₂, H₂S, and H₂O. Certain embodiments of the aforementionedelectrochemical cell and methods are described herein.

FIG. 1 schematically shows the electrochemical cell and the processwithin the present claims. Referring to FIG. 1 , the electrochemicalcell 100 includes an anode 102, a cathode 104, and a conductive hybridmembrane 106. The cell 100 also includes a “fuel” stream feed channel108 through which the fuel stream consisting mainly of H₂S and H₂O maybe supplied to the anode 102 side. Simultaneously, a feed stream 118 ofan “oxidizer” consisting mainly of CO₂ and H₂O may be supplied to thecathode 104 side of the cell through the oxidizer feed channel 116. Uponcontacting the anode, H₂S and water in the fuel feed stream undergo thefollowing oxidation reaction:

2H₂S+2H₂O+2O²⁻→2SO₂+8H⁺+12e ⁻

In essence, the oxygen ions O²⁻ that have migrated to the surface of theanode 102 from the cathode 104 through the membrane 106 (see 122 in FIG.1 ) oxidize the fuel to produce SO₂, hydrogen ions H⁺, as well as theelectric current (a number of e⁻). The stream of gaseous SO₂ 114 beingproduced in this reaction can be harvested from the anode 102 side ofthe cell 100 through the outlet channel 114. In the meantime, the H⁺produced in the oxidation reaction migrate through the conductivemembrane 106 from the anode 102 to the cathode 104 side of the cell 100(see 120 in FIG. 1 ). The electric current that is produced in theoxidation reaction also flows through the electrically conductive hybridmembrane from anode 102 to cathode 104 (flow of elections not shown). Atthe cathode 104 side of the membrane, CO₂ and water supplied through116, along with the H⁺ ions that migrated from the anode side (see 120),and with the aid of the electric current produced on the anode 102 side,undergo the following cascade of reduction reactions:

CO₂+2e ⁻→CO+O²⁻  (1)

H₂O+2e ⁻→H₂+O²⁻  (2)

8H⁺+8e ⁻→4H₂  (3)

In essence, the CO₂ and the H₂O are electrolyzed by the electric currentto produce syngas, a gaseous mixture of CO and H₂. The oxygen anions O²⁻that are produced in reactions (1) and (2) become the oxidizing reagentwhen those anions migrate to the anode 102 side (see 122). The H⁺cations are also reduced by the electric current to supply an additionalamount of molecular gaseous H₂ to the syngas mixture. The syngas 126,including the extra H₂, can be collected rom the cathode side of thecell 100 through the outlet channel 124. In addition, the electriccurrent produced on the anode side, depending on the flow rates ofstreams 110 and 118, instead of reducing CO₂ and H₂O to produce syngas,can be diverted in 128 and supplied to a local utility grid (theelectric current 128 is DC, and may be converted to AC before beingsupplied to the users of electricity).

Accordingly, the present disclosure provides a method of reacting H₂Sand CO₂, a method of co-processing H₂S and CO₂, or a method of producingsyngas (CO and H₂) and SO₂ from H₂S, CO₂, and H₂O. The method generallyincludes an electrochemical cell containing an anode, a cathode, and aconductive membrane positioned between and in electrochemical contactwith the anode and the cathode. The method further includes contactingthe anode of the electrochemical cell with a stream consisting mainly ofH₂S and H₂O. In one example, this contacting leads to producing a streamconsisting mainly of SO₂ on the anode side of the electrochemical cell.The method may further include collecting (e.g., harvesting) the streamof SO₂ for further use. Additionally, the method further includescontacting the cathode of the electrochemical cell with a streamconsisting mainly of CO₂ and H₂O to produce a stream containing mainlysyngas (CO and H₂). The method may also include collecting (e.g.,harvesting) the stream of syngas for further use. In some embodiments,contacting the anode with H₂S and contacting the cathode with CO₂ occursimultaneously.

The anode side of the electrochemical cell is generally operated in amanner applicable to operation of a fuel cell, for example, a solidoxide fuel cell (“SOFC”). In this arrangement, the H₂S is oxidized atthe anode to produce electric current, where the electricity in the formof a direct current of electrons is moving through the electricallyconductive electrolyte of the conductive hybrid membrane (as discussedabove with reference to FIG. 1 ).

Generally, the anode is made of a solid electrolyte material and iseither in direct contact with the electrolyte of the hybrid membrane, oris connected to the conductive membrane material through an interconnectlayer, such as a metallic or ceramic layer. Suitable examples of anodematerials include pure metals, metal sulfides, metal oxides,yttria-stabilized zirconia (YSZ), or any combination thereof. Suitableexamples of metal sulfides include thiospinels, such as WS₂, CoS_(1.035)or Li₂S. Suitable examples of metal oxides include LiCoO₂, V₂O₅, NiO,LaSrMnO₆, CeO₂, Y₂O₃, La₂O₃, MgO, and TiO₂. Other suitable anodematerials include metallic Ru, Cu, Co, Ni, Pt, or Ag, or any combinationthereof. Other examples of anode materials include perovskite andchromite (La_(0.8)Sr_(0.2)Cr_(0.5)Mn_(0.5)O₃). Any anode materialgenerally known to be useful in SOFC operation may be used in theelectrochemical cell of this disclosure. Anode layers in the cells ofthis disclosure may be of various thickness ranging from about 1 nm toabout 1 cm, or more, depending on the needs, the scale, and the outputof the process.

The stream containing H₂S and H₂O can generally be a liquid phase or agas phase. When the stream is a liquid, an aqueous solution of H₂S inwater, the concentration of H₂S may be from about 0.01M to about 1M. Inone example, the stream is a saturated solution of H₂S in water at theoperating temperature. When the stream is a gas, it may be applied tothe anode side of the electrochemical cell at a pressure from about 1bar to about 20 bar, from about 1 bar to about 15 bar, from about 1 barto about 10 bar, or from about 1 bar to about 5 bar. In someembodiments, the pressure of the gaseous stream containing H₂S and H₂Ois about 1 bar, about 2 bar, about 3 bar, about 5 bar, or about 10 bar.The temperature of the H₂S stream is generally a high operatingtemperature. For example, the operating temperature on the anode side ofthe cell may be from about 100° C. to about 1,500° C., from about 200°C. to about 1,200° C., from about 400° C. to about 1,100° C., from about500° C. to about 1,200, or from about 700° C. to about 1,000° C. Anysuitable molar ratio of H₂S to H₂O may be used in this fuel stream. Insome embodiments, molar ratio of H₂S to H₂O in the fuel stream is about0.1:1, about 0.5:1, about 1:1, about 1:2, about 1:5, or about 1:10.

Generally, the cathode in the electrochemical cell is made of a solidelectrolyte material and is either in direct contact with theelectrolyte of the hybrid membrane, or is connected to the membranematerial through an interconnect layer, such as a metallic or ceramiclayer. Suitable examples of cathode materials include lanthanumstrontium manganite (“LSM”), conductive ceramics, or perovskite. In oneexample, the cathode material is a perovskite material of the generaltype La_(1-x)Sr_(x)Cr_(1-y)M_(y)O₃, where M is a metal selected from Mn,Fe, Co, and Ni. Any cathode material generally known to be useful in CO₂electrolysis (including high-temperature electrolysis) maybe used in theelectrochemical cell of this disclosure. Cathode layers in the cells ofthis disclosure may be of various thickness ranging from about 1 nm toabout 1 cm, or more, depending on the needs, the scale, and the outputof the process. The cathode material may be selected to operate at hightemperatures, for example, from about 500° C. to about 1,500° C., orfrom about 800° C. to about 1,200° C.

The stream containing CO₂ and H₂O can generally be a liquid phase or agas phase. When the stream is a liquid, an aqueous solution of CO₂ inwater, the concentration of CO₂ may be from about 0.01M to about 1M. Inone example, the stream is a saturated solution of CO₂ in water at theoperating temperature. When the stream is a gas, it may be applied tothe cathode side of the electrochemical cell at a pressure from about 1bar to about 20 bar, from about 1 bar to about 15 bar, from about 1 barto about 10 bar, or from about 1 bar to about 5 bar. In someembodiments, the pressure of the gaseous stream containing CO₂ and H₂Ois about 1 bar, about 2 bar, about 3 bar, about 5 bar, or about 10 bar.The temperature of the CO₂ stream is generally a high operatingtemperature. For example, the operating temperature on the cathode sideof the cell may be from about 100° C. to about 1,500° C., from about200° C. to about 1,200° C., from about 400° C. to about 1,100° C., fromabout 500° C. to about 1,200, or from about 700° C. to about 1,000° C.Any suitable molar ratio of CO₂ to H₂O may be used in this oxidizerstream. In some embodiments, molar ratio of CO₂ to H₂O in the oxidizerstream is about 0.1:1, about 0.5:1, about 1:1, about 1:2, about 1:5, orabout 1:10.

In a general aspect, the electrochemical cell of this disclosurecontains a hybrid conductive membrane between the anode and the cathode.The membrane is hybrid in that it is both ion-conductive andelectrically conductive. For example, the ion-conductive membrane mayallow a simultaneous free flow of hydrogen cation (H⁺), oxygen anion(O²⁻), as well as the other ions between the anode and the cathode (asdiscussed for FIG. 1 discussed above). At the same time, the membrane iselectrically conductive, allowing a direct electric current(unidirectional flow of electrons) from the anode to the cathode throughthe membrane. The membrane may be water-permeable, partiallywater-permeable, or water-impermeable. In one example, the conductivemembrane may be a single layer. In another example, the conductivemembrane may have a multilayered structure, where each layer contains adifferent conductive material. The membrane may contain, for example, alayer of a liquid electrolyte, such as a NaCl solution, in between thetwo layers of solid electrolytes. The membrane may also have a segmentedstructure, where each of the segments is selectively conductive for oneion. In one example, the membrane may contain two segments: a segmentselectively conductive for H⁺ and a segment selectively conductive forO²⁻, where each segment is made from an electrically conductivematerial. In another example, the membrane may contain three separatesegments: s segment selectively conductive for H⁺, a segment selectivelyconductive for O²⁻, and an electrically conductive segment (allowingflow of e⁻ from anode to cathode) which is not conductive to H⁺ and O²⁻.Examples of mixed solid ion conductive electrolytes of the hybridmembrane include a dense layer of ceramic capable or conducting oxygenions, yttria-stabilized zirconia (YSZ), scandia stabilized zirconia(ScSZ) (e.g., about 9 mol,% Sc₂O₃—9ScSZ), gadolinium doped ceria (GDC),lanthanum strontium cobalt ferrite (LSCF), Sr-doped La manganites (LSM),and ferrites-nickelates (LSFN). Some examples of hybrid materialsinclude CaO, ZrO, and TiO, and combinations thereof. Other examplesinclude BaZr_(0.4)Ce_(0.4)Y_(0.1)Yb_(0.1) (BZCYYb—4411),BaZr_(0.4)Ce_(0.4)Y_(0.1)Yb_(0.103-d), andBaZr_(0.1)Ce_(0.7)Y_(0.1)Yb_(0.1) (BZCYYb1711), or a combinationthereof. The membrane material may be chosen to withstand the hightemperature operating conditions from about 700° C. to about 1,000° C.In some embodiments, electric conductivity of the hybrid membrane isfrom about 10⁻¹ S/m to about 10⁸ S/m (Siemens per meter). For example,electric conductivity of the hybrid membrane at the operating conditions(such as high temperature) is about 10² S/m, about 10⁴ S/m, about 10⁶S/m, or about 10⁷ S/m.

In some embodiments, molar ratio of H₂S to CO₂ in the electrochemicalcell is from about 0.1:1 to about 10:1. For example, the molar ratio ofH₂S to CO₂ is about 1:10, about 1:2, about 1:1, about 1:2, about 1:5, orabout 1:10. In some embodiments, the flow of H₂S to anode and the flowof CO₂ to cathode may be selected such that instead of a syngas, anelectric current is produced and harvested to a local utility grid. Inone example, the electric current produced from the electrochemical cellis from about 50 W to about 100 MW. In another example, the electriccurrent produced in the cell is from about 1 A to about 1,000 A, or fromabout 1 A to about 100 A. In yet another example, the electric currentis from about 1 V to about 300 V, or from about 50 V to about 250 V.

In some embodiments, the stream containing SO₂ collected from the anodeside of the electrochemical cell is substantially pure. In one example,the stream comprises no more than from about 0.5 wt. % to about 10 wt. %of H₂S or H₂O, or a combination thereof. In some embodiments, the streamcontaining SO₂ contains about 80 wt. %, about 85 wt. %, about 90 wt. %,about 95 wt. %, or about 99 wt. % of SO₂.

In some embodiments, molar ratio of CO to H₂ in the stream containingsyngas is from about 1:10 to about 10:1, from about 1:10 to about 5:1,or from about 1:5 to about 1:1. In some embodiments, molar ratio of COto H₂ is about 1:10, about 1:5, about 1:2, about 1:1, about 2:1, about5:1, or about 10:1. In one example, the syngas is collected from thecathode side of the cell at a temperature insufficient to react CO andH₂ to produce any other chemical compound. In some embodiments, thesyngas is substantially free of components other than CO and H₂. Forexample, the syngas contains no more than about 0.5 wt. %, about 1 wt.%, about 2 wt. %, about 5 wt. %, or about 10 wt. % of H₂O or CO₂, or acombination thereof.

The syngas produced according to the methods of this disclosure may beused in any field where syngas is generally useful. For example, thesyngas can be burned (reacted with O₂ at elevated temperature) toproduce lighting (commonly known as gas lighting), or it may be used asfuel for cooking and heating. In another example, syngas may be used toproduce sponge iron by reduction of iron ore. Syngas can also be usefulto produce diesel in Fischer-Tropsch process, or it may be chemicallyconverted to other useful chemicals, such as methane, methanol, ordimethyl ester.

Likewise, SO₂ produced using the methods of this disclosure may beuseful in any manner where this compound is generally useful. In oneexample, SO₂ may be used to react with H₂S to produce elemental sulfur(Claus process). SO₂ may also be reacted with Cl₂ to yield sulfurylchloride (SO₂Cl₂). Importantly, SO₂ may be oxidized by oxygen in thepresence of water to produce sulfuric acid (H₂SO₄). Various S-containingheterocyclic reagents may also be prepared from SO₂ for pharmaceuticalindustry, sulfolane being one of the examples.

OTHER EMBODIMENTS

It is to be understood that while the present application has beendescribed in conjunction with the detailed description thereof, theforegoing description is intended to illustrate and not limit the scopeof the present application, which is defined by the scope of theappended claims. Other aspects, advantages, and modifications are withinthe scope of the following claims.

What is claimed is:
 1. A method for producing syngas and SO₂ from H₂S,CO₂, and H₂O in an electrochemical cell comprising an anode, a cathode,and a conductive membrane positioned between and in electrochemicalcontact with the anode and the cathode, the method comprising:contacting the anode of the electrochemical cell with a streamcomprising H₂S and H₂O to produce a stream comprising SO₂; andcontacting the cathode of the electrochemical cell with a streamcomprising CO₂ and H₂O to produce a stream comprising syngas.
 2. Themethod of claim 1, wherein contacting the anode and contacting thecathode occur simultaneously.
 3. The method of claim 1, furthercomprising collecting the stream comprising SO₂ from the anode side ofthe electrochemical cell.
 4. The method of claim 1, further comprisingcollecting the stream comprising the syngas from the cathode side of theelectrochemical cell.
 5. The method of claim 1, wherein the streamcomprising H₂S and H₂O is a gas.
 6. The method of claim 5, wherein thepressure of the stream is from about 1 bar to about 20 bar.
 7. Themethod of claim 5, wherein the temperature of the stream is from about100° C. to about 1,500° C.
 8. The method of claim 5, wherein molar ratioof H₂S to H₂O in the stream is from about 0.1:1 to about 10:1.
 9. Themethod of claim 5, wherein temperature of the stream is from about 700°C. to about 1,000° C.
 10. The method of claim 1, wherein the anodematerial comprises WS₂, CoS_(1.035) or Li₂S.
 11. The method of claim 1,wherein the stream comprising CO₂ and H₂O is a gas.
 12. The method ofclaim 11, wherein the pressure of the stream is from about 1 bar toabout 20 bar.
 13. The method of claim 11, wherein the temperature of thestream is from about 100° C. to about 1,500° C.
 14. The method of claim11, wherein molar ratio of CO₂ to H₂O in the stream is from about 0.1:1to about 10:1.
 15. The method of claim 11, wherein temperature of thestream is from about 700° C. to about 1,000° C.
 16. The method of claim1, wherein the cathode material comprises a perovskite material of thegeneral type La_(1-x)Sr_(x)Cr_(1-y)M_(y)O₃, where M is a metal selectedfrom Mn, Fe, Co, and Ni.
 17. The method of claim 1, wherein theconductive membrane comprises a material selected from yttria-stabilizedzirconia (YSZ), scandia stabilized zirconia (ScSZ), gadolinium dopedceria (GDC), lanthanum strontium cobalt ferrite (LSCF), Sr-doped Lamanganites (LSM), and ferrites-nickelates (LSFN).
 18. The method ofclaim 1, wherein molar ratio of H₂S to CO₂ in the electrochemical cellis from about 0.1:1 to about 10:1.
 19. The method of claim 1, whereinmolar ratio of CO to H₂ in the syngas is from about 1:1 to about 1:5.20. The method of claim 1, wherein the syngas comprises no more thanfrom about 0.5 wt. % to about 10 wt. % of CO₂ or H₂O, or a combinationthereof.